Photons advance on two fronts

Portland, Ore.  Photons have electron and molecular properties that could be applied to such areas as sensors and fiber optics, recent experiments show. One group has discovered that solitons  self-sustaining waves of light  can form stable structures resembling molecules. Another team has extended photonic work on the Hall effect to phonons, which are quantized vibrations in a crystal lattice.

The first discovery could enable fiber-optic telecommunications lines to double their capacity and eliminate the need for repeaters. The second result might enable Hall-effect sensors to be made from dielectric materials.

France's Grenoble High Magnetic Field Laboratory reported observing the Hall effect in phonons. The work builds on a discovery several years ago by Grenoble researcher Geert Rikken that the Hall effect can be realized

in photons  a surprising revelation because it had been thought that only charged particles, such as electrons, would respond in such a way to an external magnetic field. Now Rikken and Grenoble colleagues Cornelius Strohm and Peter Wyder have shown that phonons can likewise be harnessed to exhibit the Hall effect.

Normally, the Hall effect occurs when an electric current flows perpendicularly to a magnetic field applied to a conductor. Under the influence of the magnetic field, the electrons begin to travel in circles, with the centers of the circles drifting perpendicularly to the current. The effect is commonly used in magnetic-field sensors.

Since photons do not carry charge, the effect that Rikken observed in that earlier experiment showed up as a thermal current rather than an electrical current. Like photons, phonons are not charged, but in the recent experiment a thermal current was found to flow perpendicularly to both the applied magnetic field and the direction of an applied current flow.

The researchers demonstrated the effect by passing the thermal current through a crystal lattice of terbium gallium garnet  a paramagnetic semiconductor used in magneto-optics  while simultaneously applying the perpendicular magnetic field. The result was an induced thermal-current flow.

The Hall effect is used widely in semiconductors for sensing and switching. A phonon-based version of the effect could lead to new operating modes for magneto-optical materials and devices that might in turn yield new sensor types.

'Molecules' of lightIn the soliton experiment, professors Martin Stratmann and Fedor Mitschke at Germany's Rostock University have demonstrated that solitons  self-sustaining optical waves that do not dissipate  can form into stable paired structures that resemble molecules. They speculate that such structures could be used to represent a third logical value in digital optical encoding, doubling the information capacity of current optical fiber.

Solitons exist in most media that exhibit waves, such as the sea waves that travel from the Pacific Coast of the United States to the sea of Japan. In fiber optics, solitons can be produced in a nonlinear dispersion-shifted fiber, where the trailing edge of each pulse moves faster than the leading edge. The result is a wave whose peak power tends to increase rather than decrease over the length of an optical fiber, yielding incredible sustaining power without repeaters.

The first soliton optical-networking systems were developed at Bell Laboratories in the 1980s. A few years ago, a Nippon Telegraph and Telephone Corp. lab demonstrated a dispersion-shifting soliton data transmission system capable of 100-kilometer distances at 640 Gbits/second with no repeaters. NTT also reported successfully sending a stream of solitons down a 1 million-km fiber-optic cable at 40 Gbits/s with no repeaters. The NTT researchers went on to create both time- and wavelength-division multiplexing systems for optical soliton communications.

The Rostock researchers' soliton molecules add further information-carrying capacity to such schemes. To build the molecules, Stratmann and Mitschke used bright solitons and dark solitons, respectively, to represent negative and positive charge (i.e., electrons and the absence of electrons, or holes). They bound two bright solitons to a single dark soliton, which served as the "glue" to form the molecule of light.

The researchers now are working to create other types of soliton molecules.

In earlier work, Mitschke and Rostock professor Soeren Rutz had shown that arrays of soliton molecules could exist in either a fluid or a solid state, which the researchers called soliton gases and soliton crystals.